1. Field of the Invention
The present invention relates to corona charging devices and, more particularly, corona charging devices that are used in electrophotographic reproduction apparatus to give the photoconductive surfaces an electrostatic charge prior to light exposure.
2. Technical Considerations and Prior Art
Generally, corona charging devices use a U-shaped wire which is preferably placed between two supports of insulating material which are inserted in the two ends of a metal screen which forms a grounded gutter. The wires have free ends which must always be perfectly taut and are connected to a high voltage source of electricity which causes the corona effect by giving rise to a current that generates a charge.
Like all fine wires subjected to a high voltage current, the wires of this device must be changed frequently because they attract dust from the air charged with tribo-electricity. This dust accumulates on the wires, modifying the properties thereof which produced local over-voltages thereon rapidly, causing charge defects. The wires, therefore, require replacement.
Replacement of these wires is a delicate and complicated operation because the wires have to be strung correctly while avoiding excess tautness which might break the wires. After they are strung, they must then be connected to the high voltage source. It is, of course, desirable to replace these wires rapidly to minimize the downtime of the apparatus using the wires. This change, therefore, requires the intervention of an experienced person, if not of a specialist.
To obtain the correct tension of the wire, the wire is wound in several turns around the smooth part of the shank of a screw which is fastened in the input support. Then the wire is strung along a path which connects it to the opposite support and is brought back to the input support where it is fastened in the same way by winding around another screw.
The wire tension is regulated by modifying the winding of the wire around these two screws. This is accomplished by screwing the screws in or backing them off. Considerable experience is required to regulate these screws suitable without breaking the wire, and the prior art approach is not immune from relaxation of friction that loosens the wire, which can cause disturbances of the charge and damage to the device. Generally, an effort is made to compensate for this relaxation by using a spring that extends from one of the ends of the wire.